1. Field of the Invention
The invention relates to a method of manufacturing secondary batteries, using electrode plates having active material layers. More specifically, the invention is concerned with a method of appropriately producing electrode mixture paste used for forming the active material layers of the electrode plates, and manufacturing secondary batteries using the electrode mixture paste.
2. Description of Related Art
A secondary battery is generally comprised of positive and negative electrode plates and a separator, which are stacked together and rolled up, to form a cell housed in a case. Each of the electrode plates includes a current-collector foil coated with active material layers. The electrode plate is produced by coating a metal foil that provides the current-collector foil, with paste containing an active material, and drying the coated foil. The paste is generally produced by mixing and kneading the active material in a powder state and a solvent in a liquid state. A technology concerned with production of the paste is described in Japanese Patent Application Publication No. 2005-285606 (JP 2005-285606 A). According to the technology described in JP 2005-285606 A, a liquid reagent is dripped into a powder of electrode active material, and kneading torque is measured. The properties of the electrode active material are evaluated based on the relationship between the amount of the dripping liquid agent, and the kneading torque. Various conditions used when producing the paste are determined based on the properties thus evaluated.
In the meantime, when a powder is mixed and kneaded with a solvent, the following case may occur. Namely, dilatancy may occur in paste while the powder and the solvent are being kneaded, depending on conditions, such as components of the powder and the speed of shearing the paste during kneading. The paste is kneaded with a rotating stirrer. If dilatancy occurs in a portion of the paste, the portion has reduced fluidity or flowability, and is liable to stick to the stirrer. In this case, the remaining portion of the paste is not sufficiently stirred. Therefore, sufficient mixing and kneading may not be accomplished. As a result, paste having adequate properties, in which the powder is favorably dispersed in the solvent, may not be produced. In addition, even if the electrode mixture paste is formed from the paste in which dilatancy occurs, it would be difficult to coat the current-collector foil with the electrode mixture paste, because of excessively high viscosity of the paste.